Top quark
The top quark, sometimes also referred to as the truth quark, (symbol: t) is the most massive of all observed elementary particles. It derives its mass from its coupling to the Higgs Boson. This coupling is very close to unity; in the Standard Model of particle physics, it is the largest (strongest) coupling at the scale of the weak interactions and above. The top quark was discovered in 1995 by the CDF and DØ experiments at Fermilab.
A collision event involving top quarks | |
| Composition | elementary particle |
|---|---|
| Statistics | fermionic |
| Family | quark |
| Generation | third |
| Interactions | strong, weak, electromagnetic, gravity |
| Symbol | t |
| Antiparticle | top antiquark ( t ) |
| Theorized | Makoto Kobayashi and Toshihide Maskawa (1973) |
| Discovered | CDF and DØ collaborations (1995) |
| Mass | 172.76±0.3 GeV/c2 |
| Decays into | bottom quark (99.8%) strange quark (0.17%) down quark (0.007%) |
| Electric charge | + 2 /3 e |
| Color charge | yes |
| Spin | 1 /2 ħ |
| Topness | 1 |
| Weak isospin | LH: + 1 /2, RH: 0 |
| Weak hypercharge | LH: +1 /3, RH: +4/3 |
Like all other quarks, the top quark is a fermion with spin 1 /2 and participates in all four fundamental interactions: gravitation, electromagnetism, weak interactions, and strong interactions. It has an electric charge of + 2 /3 e. It has a mass of 172.76±0.3 GeV/c2, which is close to the rhenium atom mass. The antiparticle of the top quark is the top antiquark (symbol: t, sometimes called antitop quark or simply antitop), which differs from it only in that some of its properties have equal magnitude but opposite sign.
The top quark interacts with gluons of the strong interaction and is typically produced in hadron colliders via this interaction. However, once produced, the top (or antitop) can decay only through the weak force. It decays to a W boson and either a bottom quark (most frequently), a strange quark, or, on the rarest of occasions, a down quark.
The Standard Model determines the top quark's mean lifetime to be roughly 5×10−25 s. This is about a twentieth of the timescale for strong interactions, and therefore it does not form hadrons, giving physicists a unique opportunity to study a "bare" quark (all other quarks hadronize, meaning that they combine with other quarks to form hadrons and can only be observed as such).
Because the top quark is so massive, its properties allowed indirect determination of the mass of the Higgs boson (see § Mass and coupling to the Higgs boson below). As such, the top quark's properties are extensively studied as a means to discriminate between competing theories of new physics beyond the Standard Model. The top quark is the only quark that has been directly observed due to its decay time being shorter than the hadronization time.